Sunday, October 15, 2006

Earlier this week I foundCarl Burch's wonderful Logisim which does digital logic simulation. It took a few hours of tweaking, and one flash of insight (below)... but here is what a single BitGrid cell looks like in an idealized format.

The cell consists of a single 16x4 RAM cell (4 bits address, 4 bits data). I used a ROM in the simulation to allow it to persist across saves, and simplify the layout.

There are any number of ways you could wire this thing up... the flash of insight I had was that I wanted it to be very simple to turn a cell into a simple pass-through repeater. I figured that if addresses 0-F were programmed with contents 0-F, and it just worked that way... it would be easiest to understand. This leads naturally to the layout you see pictured here.

If you want to see for yourself, here is the Logisim Circuit file. (You'll have to save it, then rename it to *.circ for Logisim due to limitations of my web host at 1and1.com)